1. Field of the Invention
This invention relates to an optical recording medium adapted to record information in a recording layer by irradiating a light beam thereto to induce a crystallographic change. More particularly, it relates to a method for preparing the optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable optical recording medium is of the phase change type wherein laser light is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by a single light beam and operated by a drive unit with a relatively simple optical system as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Ge--Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe.sub.2 phase.
For prior art phase change type optical recording media, recording layers are formed using vacuum deposition equipment and remain amorphous immediately after formation. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; illuminating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; illuminating flash light to the substrate to achieve pseudo-crystallization by so-called photo-darkening, which method takes advantage of the photo characteristics of calcogen compounds, as disclosed in JP-A 281219/1992; and high-frequency induction heating the medium. JP-A 98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization step by laser beam illumination takes a long time and is makes the productivity lower. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing errors in tracking. The method of illuminating flash light is also low in productivity because several shots of illumination are necessary to achieve full crystallization.
Under the circumstances, the use of a so-called bulk erasing is the only technique which is regarded commercially acceptable and currently used. The bulk eraser illuminates a beam from a high power gas or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
The bulk eraser, however, requires a time of several minutes for initializing optical recording discs of 12 cm in diameter. Then the initializing step is a rate-determining step in the making of optical recording discs. While TeGeSb base materials are currently most widely used for phase change recording layers, it is believed that the initializing operation cannot be removed insofar as these materials are used.